The invention relates to digital telecommunication systems and more particularly to a transmission testing arrangement for use with such a telecommunication system.
Digital telecommunication systems known in the art provide for temporary communication links between selected ones of a multitude of peripheral devices, such as subscriber stations under control of a central switch. Preferably, the central switch is of modular structure establishing different control levels. Transmission lines to peripheral devices and possibly other switches are grouped together and connected to a respective one of sub-units of the switch, the so-called line trunk groups. Each line trunk group is a control device designed for establishing information transfer along the transmission lines connected thereto through respective subscriber line module interfaces. Cross-connections between transmission lines associated with one of the same line trunk group are directly established under control of such line trunk group by means of an integral part of it, a group switching network. Cross-connections between transmission lines belonging to different line trunk groups are achieved by means of a central switching network having input/output ports connected to a respective one of the line trunk groups.
A common control unit is provided in the central switch for controlling all switching operations under real-time conditions, especially when the operation of different modules of the central switch is utilized. The common control unit is constructed of a programmable processor system having also input/output units residing together with a central processor on a system bus and being designed for transferring control information to and from the described modules of the central switch.
It is one characteristic of pulse code modulated (PCM) switching systems, in contrast to former analog switching systems, that information transfer is no longer achieved by means of physically through-connected lines rather than by applying time division methods, whereby the conditions on the analog subscriber lines are successively sampled and formatted into data words, so-called code words. These code words are arranged in a pulse frame together with control information. The pulse frame is processed word by word by the central switch is synchronism with a time frame of the system. This approach necessitates a great deal of synchronization among the different modules of the switch and a close control in order to assure the desired fail-safe operation. Consequently, even more than with a conventional switch, measures have to be taken that any malfunction of parts or modules of the telecommunication system is recognized even before failures become evident to a user or subscriber of the system.
A variety of stand-alone testing equipment is known which comprises tools used by maintenance personnel for measuring transmission characteristics or monitoring the performance of a telecommunication system. Conventionally, such testing equipment used to be of analog design. By this time, it became more and more sophisticated and was replaced by digital hardware structures with processing capability. However, such testing equipment is of less concern with respect to the present invention. The present invention, rather more, is directed to a self-testing arrangement which can easily be tailored to the needs of a specific digital telecommunication system and may be even made a built-in feature. Insofar, such self-testing facility is not directly a substitute for the testing equipment mentioned above, it is rather an additional means designed to facilitate routine tests automatically initiated by common control in order to monitor the performance of the telecommunication system and its various sub-units.
The complexity of a large digital telecommunication system including the central switch and the amount of inter-action and cooperation of various sub-units creates the need for extensive self-testing. This could be also achieved by a design where the transmission and switching modules include sophisticated internal fault detection in hardware. However, this approach is very costly and it is, therefore, assumed that most of those modules do not provide for such a hardware facility. Instead common control includes a scheduler of the central processor which is designed to initiate various tests periodically. The results of such tests give an overview of the performance of the entire system and may demand some automatic reconfiguration by taking redundant parts of the system out of service in case of a suspected or even actual malfunction. Only then, maintenance personnel will take action to further investigate the operation of such a module or device and to physically replace it, if necessary.